The present invention generally concerns a CCD (Charge Coupled Device) and in particular an CCD image sensor for temporarily fixing a picture on a video screen.
The CCD is a kind of charge transfer device that is a dynamic element for transferring charges through a given channel under the control of the clock pulses.
Transferring in series the charges injected by input diodes from one end to the other end, the CCD is used as a shift register in a memory device, as well as serving a variety of logical functions, signal processing, image processig, etc. In image processing, the CCD is semiconductor device arrayed so that a flux of charges is formed in proportion to the intensity of light and transferred to a detecting device to read the data. To this end, interline transfer type CCD (IT-CCD) is used.
Referring to FIG. 1A for illustrating the structure of picture elements of the IT-CCD, there are disclosed a photo diode region 20 for producing charges in response to the external light, vertical CCD region 22, and transfer gate region 21 for transferring the charges to the CCD. In the drawing, the direction of the charge transferring in the vertical CCD region 22 is coming out perpendicularly to the cross section of the structure. As shown in the drawing, in an n-type substrate 10 is formed p-type well 11, where there are formed a region 12 for inducing n+ charges in the photo diode region 20, a region 14 for transferring the induced charges to the vertical CCD region 22, n-channel region 13 of the vertical CCD region 22, and a channel stopper 15 of p+ type. Further, in oxide film over the substrate 17 is formed a polysilicon gate 16 to overlap the transfer gate region 21 and the CCD region 22.
Thus, the charges induced by light in the n+ region 12 of the photo diode region 20 are transferred through the region 14 of the transfer gate region 21 to n-channel region 13 of the vertical CCD region 22 according to the control signal applied to the polysilicon gate 16, finally transferred through the vertically defined path of the vertical CCD region 22.
Referring to FIG. 1B for illustrating the conventional IT-CCD type solid-state image sensor including the picture elements as shown in FIG. 1A, there is disclosed a plurality of photo diodes 20 arranged in a matrix structure, between which columns is arranged the vertical channel 22 to vertically transfer the charges under the control of vertical clock pulses .phi..sub.V1, .phi..sub.V2, .phi..sub.V3, .phi..sub.V4 applied to the polysilicon gate. Between the photo diodes 20 and the vertical channel 22 is positioned the transfer gates 21. An horizontal CCD channel 23 horizontally transfers the charges coming out of the vertical CCD channel 22 to an output buffer 24 under the control of horizontal clock pulses .phi..sub.H1, .phi..sub.H2.
The transfer gate 21 is positioned under the polysilicon gate 16 of the vertical CCD channel 22 to form the channel region 14 (Refer to FIG. 1A) for transferring the charges formed in the photo diodes to the vertical CCD channel 22, which transfer gate 21 serves a MOS transistor operated by the polysilicon gate.
Referring to FIG. 1B, the vertical clock pulses .phi..sub.V1, .phi..sub.V2, .phi..sub.V3, .phi..sub.V4 and horizontal clock pulses .phi..sub.H1, .phi..sub.H2 are the control signals respectively applied to the polysilicon gates of the vertical and horizontal CCD channels 22 and 23 via the charge transfer directions.
In such a IT-CCD solid-state image sensor, the charges formed by light in the photo diodes 20 are transferred to the vertical CCD channel 22 via the transfer gate 21 positioned under the polysilicon gate having the vertical clock pulses applied. The vertical CCD channel 22 vertically transfers the charges to the horizontal CCD channel 23 according to the vertical clock pulses, while the horizontal CCD channel horizontally transfers the charges to output buffer 24 under the control of the horizontal clock pulses.
Conventionally, in order to temporarily fix a desired one of the moving pictures displayed on the screen of a CRT by employing such a solid-state image sensor is used a separate memory device external to the sensor through an A/D and D/A (Analog-to-Digital/Digital-to-Analog) converter, or otherwise the content recorded on a magnetic tape in a VTR is repeatedly reproduced. However, such methods require an externally separate equipment to fix a picture of high resolution, so that it is difficult to make them available.